Electrically variable sweep circuit



Nav. s, 1960 P. F. SMITH ELECTRICALLY VARIABLE SWEEP CIRCUIT Filed April 24, 1957 OUTPUT M" ALTERNATE OUTPUT INVENTOR- je/ guv /-T SM/ TH c/i Tree/v5! United States Patent ELECTRICALLY VARIABLE SWEEP CIRCUIT Perrin F. Smith, Saratoga, Calif., assignor to International Business Machines Corporation, New York, N.Y., a corporation of New York Filed Apr. 24, 1957, Ser. No. 654,793

1 Claim. (Cl. 328-185) This invention relates in general to waveform generators and in particular to an improved sweep voltage generator.

An object of the present invention is to provide an improved sweep voltage generator.

Another object of the present invention is to provide a sweep voltage generator including a constant current generator which is linear throughout the sweep cycle.

A further object of the present invention is to provide a sweep voltage generator where the reference potential for the generated sawtooth voltage wave is independent of the characteristics of the constant current generator.

A still further object of the present invention is to provide a sweep voltage generator whose sweep rate may be varied rapidly in response to a control voltage.

Other objects of the invention will be pointed out in the following description and claim and illustrated in the accompanying drawings, which disclose, by way of ex ample, the principle of the invention and the best mode which has been contemplated of applying that principle.

The single figure is a schematic view of a sweep voltage generator embodying the present invention.

Referring to the drawing, the waveform generator illustrated therein comprises generally a condenser and a constant current generator 11 which includes a cathode follower 12 and resistor 13. As shown in the drawing, cathode follower 12 comprises a plate 14 connected to terminal 15 of condenser 10, a cathode 16 connected to resistor 13 at terminal 17, and a grid 18 for controlling the rate at which charging current flows through cathode follower 12 to condenser 10 in accordance with the bias voltage applied to grid 18. The other end of resistor 13 is connected to B terminal 20.

The bias voltage for grid 18 is obtained from a voltage divider 21 comprising resistors 22 and 23, grid 18 being connected to common terminal 24 between the resistors. Voltage divider 21 is connected at one end to B terminal and at the other end to a terminal 26.

Means are further providedfor setting the bias voltage E to a predetermined value to cause the condenser to be charged at a predetermined nominal rate. This means acts to establish the potential of terminal 26 and hence the voltage across voltage divider 21 with the result that bias voltage E applied to grid 18 is determined. As shown, this means comprises a differential amplifier 30 comprising triode amplifier 31 and cathode follower 32.

Triode amplifier 31 comprises a plate 33 connected to ground terminal 34 through a plate load resistor 35, a cathode 36 connected to B- terminal 20 through cathode resistor 37 and a grid 38 connected to terminal 17 of the constant current generator 11. Plate 33 connects to plate resistor 35 at terminal 26 so that the potential of terminal 26 is established by the bias voltage between grid 38 and cathode 36. Since grid 38 is maintained at some relative constant voltage (determined by terminal 17), adjustments in grid bias are obtained by adjusting the potential of cathode 36. The means for adjusting the cathode potential of triode 31 in this instance comprises potentiometer 39 and cathode follower 32. Cathode follower 32 comprises a plate 40 which is connected to ground terminal 34, a cathode 41 which is tied to cathode 36 of triode 31, and a grid 42 which is connected to movable contact 43 of potentiometer 39 through resistor 44, potentiometer 39 being connected between B terminal 20 and ground terminal 34.

The rate at which condenser 10 is charged may therefore be adjusted by the setting of potentiometer 39. This is readily seen since the setting of the potentiometer 39 determines grid bias E for cathode follower 32 which, in turn, determines the potential of cathodes 41 and 36, which establishes grid bias E for triode amplifier 31. Grid bias E in turn, determines the potential of terminal 26 and hence grid bias E for cathode follower 12, which controls the rate of current flow to terminal 15 of condenser 10.

The waveform generator also includes suitable means for controlling the period during which condenser 10 is charged at its nominal rate. Preferably, this means comprises a triode 47 which may be biased to cutoff condition in response to a gating pulse. As shown, triode 47 comprises a plate 48 connected to a suitable source of plate voltage represented by B+ terminal 49, a cathode 50 connected to terminal 15 of condenser 10, and a grid 51 connected to terminal 52 through a condenser 53. Grid 51 is normally biased slightly positively with respect to cathode 50 by means of relatively large resistor 54 connected to B+ terminal 49 so that triode 47 is normally conducting except during the period when a negative gating pulse is applied to terminal 52 to drive the grid 51 to the cutofi condition. As grid 51 reaches cutoff, electron current flows to condenser 10 and causes the potential of terminal 15 to increase in the negative direction to provide the sawtooth voltage wave.

In order to insure that the beginning of each sawtooth voltage wave starts at the same voltage value, terminal 15 of condenser 10 is clamped to ground by diode clamp 60. Diode clamp 64) comprises a cathode 61 connected to ground terminal 34 and a plate 62 connected to terminal 15 of condenser 10. Any tendency of terminal 15 of condenser 10 to rise above the potential of terminal 34 due to gating triode 47 causes diode 60 to conduct, thereby preventing such tendency.

It should also be noted that toward the end of each sweep cycle plate 14 of cathode follower 12 becomes more and more negative as a result of the electron current fiow to condenser 10. This tends to reduce the current through cathode follower 12 because plate current in a triode is not substantially independent of plate voltage. This effect is partially compensated for by simple cathode follower action, but still contributes to undesired variations in the nominal constant current through cathode follower 12. However, since grid 38 of triode amplifier 31 is connected to terminal 17, bias voltage E decreases. Triode 31 and cathode follower 32, acting as a differential amplifier, cause bias voltage E of cathode follower 12 to be increased until grid 33 is biased in accordance with the bias potential of grid 42. As a result, very good linearity is obtained throughout the complete sweep cycle.

Terminal 15 of condenser 10 is also connected directly to an output terminal 65 or, alternatively, to an output terminal 65' through a cathode follower 66. The sawtooth voltage wave is obtained from either terminal depending on the external circuitry.

In addition to providing a very simple and inexpensive sweep voltage generator having a relatively good linear sweep characteristic, the circuit shown in the drawing has the prime advantage of being able to rapidly vary the rate at which condenser 10 is charged in response to a control voltage applied to terminal 67. A control voltage pulse applied to terminal 67 varies the charging current to condenser 10, positive voltages causing an increase in current flow through cathode follower l2 and negative voltages a decrease in current flow. This is a decided advantage when it is necessary to rapidly vary the speed at which a cathode spot moves across the face of a cathode ray tube.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to the preferred embodiment, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated and in its operation may be made by those skilled in the art without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

What is claimed is:

A Waveform generator comprising in combination a condenser, a cathode follower having a plate connected ito said condenser, a grid and a cathode adapted to supply charging current to said condenser at a rate determined by the bias voltage between said grid and said cathode, a diiferential amplifier having a pair of input terminals and an output terminal, means connecting said output terminal to said grid, means connecting one of said input terminals to said cathode, and means for supplying a predetermined control voltage to the other said input terminal.

References Cited in the file of this patent UNITED STATES PATENTS 2,266,516 Russell Dec. 16, 1941 2,689,911 Stabler Sept. 21, 1954 2,769,904 Ropiequet NOV. 6, 1956 2,789,225 Fleming Apr. 16, 1957 2,852,717 McCurdy Sept. 16, 1958 

